Rapid Weathering and Erosion of Mudstone Induced by Saltwater Migration near a Slope Surface

Abstract

Barren badlands lacking vegetation and consisting of sharp-edged ridges and v-shaped gullies are widely formed by rapid incision on silt−clay fine sediments, especially in weakly consolidated mudstones around the world. The development of these badlands has been explained in terms of the repeated drying and wetting of rocks, and high salt contents in pore water. Water contents and salt concentration were monitored, for the first time, by measuring dielectric permittivity and pore water electrical conductivity (EC) beneath a badland slope in southern Taiwan. The rock slope is composed of Plio-Pleistocene marine mudstone and has a subtropical climate with distinct dry and wet seasons. The monitoring revealed that salt water at depth migrated to the slope surface in the dry season and deposited salt in desiccation cracks. During the following rainy season, rainwater dissolved the salts, diluted pore water near the slope surface, and closed desiccation cracks, consequently slowing the downward movement of water. The combined wetting of rocks and dilution of pore water near the slope surface deteriorated the rock and dispersed rock-forming grains, ultimately leading to erosion rates as high as 9 cm during a single rainy season. Subsequent drying near the slope surface and the movement of salt water from depth led to the renewal of the cycle during the following dry season.